bionic: libc: avoid -Wdeprecated-declarations via std::atomic_init

std::atomic_init is deprecated in C++20, and is slated for removal in C++26.
Replace the usage of std::atomic_init with std::atomic_store_explicit with
std::memory_ordering_relaxed.

Link: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0883r2.pdf
Link: https://github.com/llvm/llvm-project/commit/56aac567acfd696f54163e33d8df02dc2ad3a72e
Test: mmma bionic
Change-Id: Idf42aea193cfacf8dd7f8528560a396c6064468c
diff --git a/libc/bionic/pthread_rwlock.cpp b/libc/bionic/pthread_rwlock.cpp
index 6f3c6fe..92134b4 100644
--- a/libc/bionic/pthread_rwlock.cpp
+++ b/libc/bionic/pthread_rwlock.cpp
@@ -247,7 +247,7 @@
     }
   }
 
-  atomic_init(&rwlock->state, 0);
+  atomic_store_explicit(&rwlock->state, 0, memory_order_relaxed);
   rwlock->pending_lock.init(rwlock->pshared);
   return 0;
 }